The invention relates to a device for integrating a compact RF antenna held for transmitting and receiving radio frequency signals, into a wireless lighting control system installed in a standard electrical wall box. Other antennas may generally relate to this field. For example, the following U.S. patents generally relate to the field: U.S. Pat. No. 4,138,684 to Kerr; U.S. Pat. No. 4,939,702 to Urbish et al; U.S. Pat. No. 5,079,559 to Umetsu et al; U.S. Pat. No. 5,400,041 to Strickland; U.S. Pat. No. 5,982,103 to Mosebrook et al, wherein the disclosures of all of these patents are hereby incorporated herein by reference.
A good or effective antenna can create an electromagnetic (EM) field in space. This antenna will have a radiation resistance that is much larger than the loss resistance. In that way, radio frequency (RF) power is dissipated into space as opposed to dissipated in the antenna materials.
In addition, an effective antenna presents suitable impedance to the RF interface circuitry which can usually be at 50 ohms resistance with nearly 0 ohms reactive. This antenna impedance is measured using a network analyzer.
Furthermore, an effective antenna prevents standing waves on the host device by minimizing common mode currents. With the case of a wall switch, the need for low common mode currents is increased so that connections to the device do not cause excessive performance variation.
In most cases, the connection between RF circuits consists of an unbalanced transmission line, such as for example a coaxial cable, or a microstrip transmission line. Unbalanced transmission lines ideally carry differential mode currents only. However because an antenna inherently interacts with the environment, it can excite currents on the ground if not interfaced properly. This can lead to common mode currents.
Proper interfacing will match the antenna to the transmission line. A balanced antenna like a dipole will often include a Balanced to Unbalanced transformer (a BALUN). Other antennas like an “F” antenna naturally interfaces with unbalanced lines.
Common mode currents result in that an unintended voltage is established somewhere besides the antenna. One effect is that the performance can be sacrificed because power can be lost as the voltage induced radio waves are radiated inside of the appliance. Alternatively, destructive interference is established resulting in a highly irregular radiation pattern.
The design of the present invention is in contrast to previous designs which might relate to a traditional F antenna. A traditional “F” antenna naturally interfaces to the unbalanced transmission line by establishing a ground at the surface of the device. There is radiation resistance in the interval between the feed point and the open ended transmission line and the interval between the feed point to the ground connection that establishes the input impedance.
In contrast, the present invention overcomes some of the limitations of the prior art by providing for example, a modified “F” antenna that results in an antenna positioned on a different plane from an antenna feed point.